The present invention relates to a jitter measurement method and a jitter measuring apparatus for measuring the variation (jitter) in the time required for each scan line of a galvano scanner that scans a light beam on a surface to be scanned by periodically turning a reflection mirror in a reciprocating manner, and also relates to image forming apparatuses such as printers, copying machines and multi-function machines comprising the main components of the jitter measuring apparatus.
In a laser scanner head type laser printer, light from a laser light source is converted into parallel light by a collimator lens, and a polygon mirror is rotated to scan the parallel laser light across a print width on a photoreceptor. In such a laser printer, the variation in the scan time that is jitter when scanning a surface to be scanned occurs due to a manufacturing error in a deflecting surface of an optical deflector constituting a scanning optical system, a drive error in a driving mechanism of the optical deflector, etc. By accurately measuring the variation in the scan time, it is possible to easily find the cause of the scanning variation, and it is possible to manufacture scanning optical systems capable of accurately scanning light for laser printers, copying machines, multi-function machines, etc.
FIG. 1 is an explanatory view schematically showing a structural example for measuring the variation (jitter) in the scan time of a conventional laser printer. Detectors A and B that are optical sensors are mounted near both ends of a scan line of a light beam, and scanning is performed by reflecting the light beam from a laser diode LD by a polygon mirror 36 that rotates in one direction at a uniform speed. At this time, for example, the detectors A and B output detection signals SA1 and SB1, respectively, for a scan line of the first mirror surface, and then each time the mirror surface reflecting the light beam changes, the light beam scans a new scan, and detection signals SA3, SB3, SA5, SB5 . . . are outputted. Since the detection signals are separately outputted from the detectors A and B as shown in FIG. 2(a), (b), the scan time is time t1, t2, t3 from the detection signal of the detector A to the detection signal of the detector B, and the variation in the scan time can be found based on these times.
In recent years, there has been an advancement in the development of galvano scanner type laser printers using a micro mirror (micro swing mirror element), which is a mirror constructed in small size, for a reflection mirror instead of a polygon mirror as a light beam scanning mechanism. The micro mirror is realized by constructing a small-size mirror as a mechanical element for reflecting light by using a MEMS (Micro Electro Mechanical System) technology that realizes various kinds of small-size mechanical elements by the application of techniques in the semiconductor manufacturing processes. Since the micro mirror is reciprocally turned by an electrostatic force and changes the light reflection path in accordance with the turned angle, it is suitable as a light beam scanning mechanism, and systems for scanning a light beam using a micro mirror have been developed (see, for example, Japanese Patent Application Laid-Open No. 11-305159(1999)).